1. Technical Field
The present invention relates to a circuit device, an oscillator, an electronic apparatus, a vehicle, and the like.
2. Related Art
In the related art, an oscillator such as an oven controlled crys(X)tal oscillator (OCXO) or a temperature compensated crys(X)tal oscillator (TCXO) is known. For example, the OCXO is used as a reference signal source in a base station, a network router, a measurement apparatus, and the like.
The oscillator such as the OCXO or the TCXO requires a high frequency stability. However, there is a problem that a secular change which is called aging occurs in an oscillation frequency of the oscillator and thereby the oscillation frequency varies with time elapse. For example, in a case where a reference signal such as a GPS signal is not receivable and enters a so-called holdover state, the oscillation frequency varies due to the aging. Aging correction is provided as a method of suppressing the variation of the oscillation frequency. For example, a technique is disclosed in JP-A-2017-123628 as a technique of related art of the aging correction.
In JP-A-2017-123628, in a non-holdover state in which a reference signal is receivable, a loop filter of a PLL circuit outputs frequency control data to an oscillation signal generation circuit, the frequency control data is input to a Kalman filter, and the Kalman filter estimates a truth value of the frequency control data and a slope of a time change. When the reference signal is changed to the holdover state in which the reference signal is not receivable, the Kalman filter holds the estimated values of the truth value and the slope of the time change at the time of changing into the holdover state, and an aging correction unit generates the frequency control data subjected to the aging correction based on the estimated value and outputs the frequency control data to the oscillation signal generation circuit.
In a case where the above-described aging correction is performed, or in a case where a variation of an oscillation frequency due to the aging is desired to acquire as data, it is necessary to monitor the variation of the oscillation frequency. The variation of the oscillation frequency due to the aging is a long-term variation, and a slope of a time change is small. There is a problem that it is difficult to follow the long-term variation in a short time and to estimate a time change of a small value with a high accuracy.